feat(settings): expand AI config with built-in model toggle and custom providers

• Add endpoints to toggle, hide, and restore built-in provider models (gemini/openai)
• Add CRUD endpoints for custom AI providers and their associated models
• Introduce new schemas: AiModelToggle, AiBuiltInModelDelete, AiProviderCreate/Response, AiCustomModelCreate/Response/Update
• Add empty __init__.py files for nodes_lite and nodes_message packages